Electronic security systems have been proposed for detecting the unauthorized removal of articles from an area under protection. Such systems have been proposed especially for use in retail stores to prevent the theft of articles from the store and to minimize the considerable losses occasioned by shoplifting. Such systems generally include an electromagnetic field provided in a controlled area through which merchandise must pass in leaving the store. A resonant circuit is attached to articles of merchandise and the presence of the resonant circuit in the controlled area is sensed by a receiving system to denote the unauthorized removal of an article. The resonant circuit is removed by store personnel or can be electronically altered such that an article properly leaving the store can pass through the controlled area without alarm activation.
A particularly effective electronic security system is described in copending application Ser. No. 214,361, filed December 30, 1971 now U.S. Pat. No. 3,810,147, by the same inventor as herein, in which a multi-frequency resonant tag circuit is provided having a first frequency to permit detection by electromagnetic interrogation of the tag and a second frequency to permit the deactivation of the tag by an applied electromagnetic field which destroys the resonance of the tag circuit at its detection frequency. The present invention is an improvement over the system of the copending application and provides enhanced discrimination of intended signals provided by a detected tag circuit from noise and spurious signal conditions which may be present in an operating environment.